1. Field of the Invention
The present invention relates to an information acquiring apparatus and an information acquiring method for acquiring information on a specimen by using terahertz wave.
2. Description of the Related Art
Terahertz wave consists of electromagnetic waves within at least part of a frequency band of 30 GHz or higher and 30 THz or lower. Terahertz time-domain spectroscopy (THz-TDS) is one of known methods of spectroscopy in which terahertz wave is used. In THz-TDS, terahertz wave is detected while the timing of an ultrashort pulse reaching a detector is changed, whereby a temporal waveform of the terahertz wave is acquired.
Such a method of THz-TDS has been applied to apparatuses and so forth in each of which information on a specimen is acquired and an image is formed by using the acquired information on the specimen. Examples of such an imaging apparatus include a reflection THz-TDS apparatus that detects radiation reflected by a surface of a specimen or an interface in the specimen.
As disclosed by Japanese Patent Laid-Open No. 2011-112548, a reflection THz-TDS apparatus employs a method in which a specimen is brought into contact with a plate-like member and terahertz wave is applied to the specimen through the plate-like member. Using the plate-like member makes the specimen flat. Furthermore, the terahertz wave that has been reflected by the front surface of the plate-like member is measurable at each of different points of application of the terahertz wave. Therefore, changes in the intensity of the terahertz wave can be standardized, which is suitable for imaging performed in a measurement of a wide area.
In Japanese Patent Laid-Open No. 2011-112548, information on the specimen is acquired by using the amount of change in the phase of the terahertz wave that has been reflected by the interface between the plate-like member and the specimen. In some cases, the amount of change in the phase may be much smaller than the phase difference caused by the terahertz wave that has traveled through the plate-like member.
In such cases, the accuracy in the acquisition of information on the specimen can be improved under the following conditions: to accurately know the thicknesses of the plate-like member at different points of application that are to be used in the compensation for the phase difference, to perform measurement by using a plate-like member having high parallelism with a tolerance of 1 μm or smaller, and so forth. However, even if such conditions are satisfied, it is not easy to completely eliminate the phase difference caused by the terahertz wave that has traveled through the plate-like member.